1. Field of the Invention
This invention relates to a method and apparatus for staged combustion of a fossil fuel which has the capability of reducing the formation of nitrogen oxides (NOx) while simultaneously providing substantially complete combustion at low excess oxidant, that is, an overall stoichiometric ratio that does not exceed 1.25, although higher or lower ratios may be employed, if desired. More particularly, this invention relates to a burner for boilers and other process heating equipment such as fluid heaters, furnaces, radiant tubes, and kilns which are fueled by gaseous or liquid fuels, in which the fuel and/or oxidant are introduced in stages, and combustion products are substantially recirculated within the combustion zone.
2. Description of Prior Art
Conventional combustion of fossil fuels produces elevated temperatures which promote complex chemical reactions between oxygen and nitrogen, forming various oxides of nitrogen as byproducts of the combustion process. These oxides, containing nitrogen in different oxidation states, generally are grouped together under the single designation of NOx. Concern over the role of NOx and other combustion byproducts, such as sulfur oxides, carbon monoxide, total hydrocarbons and carbon dioxide, in xe2x80x9cacid rainxe2x80x9d and other environmental problems has generated considerable interest in reducing the formation of these environmentally harmful byproducts of combustion.
Known methods of combustion for reducing NOx emissions from combustion processes include flue gas recirculation and staged combustion. U.S. Pat. No. 4,004,875 teaches a low NOx burner for combustion of liquid and gaseous fuels in which the combustion area is divided into at least two stages and combustion products are recirculated, cooled and reintroduced into the primary combustion zone, resulting in a reduction of NOx emissions. The secondary combustion air is introduced into a secondary combustion zone downstream of the primary combustion zone in an amount sufficient to complete combustion therein. Fuel and primary combustion air are introduced into a primary combustion zone formed by a burner tile which provides a high temperature environment for the fuel and air mixture to promote combustion. Except for the opening into the secondary combustion zone, the tile is completely surrounded by a steel enclosure forming an annular space around the tile. Thus, as fuel and air are injected into the primary combustion zone, part of the partially combusted fuel and air is recirculated around the outside of the tile in the annular space between the tile and steel enclosure and back into the upstream end of the primary combustion zone. U.S. Pat. No. 5,350,293 teaches a combustion process and apparatus employing air staging in which at least two combustion zones, a primary combustion zone and a secondary combustion zone disposed downstream of the primary combustion zone, are formed. In addition, at least a portion of the products of combustion generated in the primary combustion zone are recirculated from a downstream region of the primary combustion zone to an upstream region thereof. U.S. Pat. No. 5,573,391 and U.S. Pat. No. 5,636,977 teach a multi-stage combustion process and apparatus in which internal recirculation of combustion products is carried out only in the secondary combustion zone and a portion of the primary, fuel-lean air/fuel mixture is introduced into the secondary combustion zone. See also U.S. Pat. No. 4,629,413 which teaches a low NOx burner utilizing staged combustion in which a mixture of primary combustion air and fuel is introduced into a primary combustion chamber and secondary combustion air is introduced into the combustion chamber in a manner such that the mixing of the secondary combustion air with the flame generated by the mixture of fuel and primary combustion air is delayed; U.S. Pat. No. 5,044,932 which also teaches a process and apparatus for reducing the NOx content of the flue gas effluent from a furnace in which cooled flue gases are internally recirculated from the downstream end of the combustion chamber into the upstream end of the combustion chamber where it undergoes reaction with the flame generated by the fuel and air introduced into the upstream end of the combustion chamber; U.S. Pat. No. 4,575,332 which teaches staged combustion in a swirl combustor with forced annular recycle of flue gases to the upstream end of the primary combustion zone; and U.S. Pat. No. 4,395,223 which teaches staged combustion with excess air introduced into the primary combustion zone with additional fuel being introduced into the secondary combustion zone.
It is also known that, in addition to limiting the oxygen available in a combustion process for formation of NOx emissions, NOx emissions may also be controlled by maintaining the temperature in the combustion zone below the temperature required for formation of significant amounts of NOx, about 2600xc2x0 F. Cooling of the products of combustion is suggested by U.S. Pat. No. 4,004,875 discussed hereinabove. However, by recirculating the cooled partial combustion products from the downstream of the primary combustion zone to the upstream end of the primary combustion zone, any heat removed from the primary combustion zone as a result of cooling is reintroduced into the secondary combustion zone, resulting in no net heat removal from the combustion process. Consequently, the temperatures in the primary and secondary combustion zones are not maintained below the level required for significant NOx formation.
In spite of numerous advances toward reducing NOx emissions from boilers and other process heating equipment, such as fluid heaters, furnaces, radiant tubes and kilns, which are fueled by gaseous or liquid carbonaceous or hydrocarbon fuels, a substantial amount of NOx continues to be produced by such heating equipment. As a result, there continues to be a need for further reducing the amount of NOx produced by such heating equipment, particularly if future standards restricting NOx emissions are to be met.
It is also apparent that the number of boilers and process heating apparatuses producing NOx emissions is very large and that replacement of these apparatuses with more up-to-date equipment as a means for reducing NOx emissions is not practical. As a result, proposed solutions for reducing NOx emissions must be able to address issues relating to the age and physical condition of the boilers and process heating equipment, such as the lack of gas-tight seals between various portions thereof which may result in the undesirable intake of air from around the heating equipment. For boilers and process heating equipment which lack a gas-tight seal between the combustion zone and convective pass, of which tangent-tube boilers are a typical example, combustion products from the primary combustion zone can leak into the convective pass. In addition, proposed solutions should be suitable for retrofitting to the boilers and process heating equipment, regardless of the physical condition of the tube walls.
As previously stated, combustion processes and apparatuses in which oxidantxe2x80x94typically air, oxygen, or oxygen-enriched airxe2x80x94is introduced into the processes and apparatuses in two or more stages are well known as means for reducing NOx emissions. However, in oxidant staging systems applied to boilers and process heating equipment lacking gas-tight seals between the combustion zone and the convective pass as previously discussed, the combustion products from the primary combustion zone that leak into the convective pass may contain high levels of CO, which can result in unacceptably high CO in the stack.
Accordingly, it is one object of this invention to provide a method and apparatus for combustion of fossil fuels which is capable of reducing NOx emissions compared to conventional methods and apparatuses.
It is another object of this invention to provide an apparatus for combustion of fossil fuels which is suitable for retrofitting to conventional boilers and process heating equipment.
These and other objects of this invention are addressed by a combustion apparatus comprising at least one combustion chamber wall forming at least one burner opening and enclosing a combustion chamber, and at least one multi-stage fuel/multi-stage oxidant burner having a fuel and oxidant inlet end and a fuel and oxidant outlet end attached to the at least one combustion chamber wall. At least a portion of the fuel and oxidant outlet end of the burner extends through the at least one burner opening into the combustion chamber.
The at least one multi-stage fuel/multi-stage oxidant burner comprises a first-stage plenum chamber wall enclosing a first-stage plenum chamber and forming a first-stage inlet for a first-stage fuel and a first-stage oxidant and a first-stage outlet for the first-stage fuel and the first-stage oxidant. The first-stage outlet is in fluid communication with the at least one burner opening. An end wall is disposed within the first-stage outlet having a periphery in contact with the first-stage plenum chamber wall and forming at least one first-stage nozzle opening and at least one second-stage nozzle opening. The at least one first-stage nozzle opening provides a fluid communication between the combustion chamber and the first-stage plenum chamber. A second-stage plenum chamber wall encloses a second-stage plenum and forms a second-stage inlet for a second-stage fuel and a second-stage oxidant and a second-stage outlet for the second-stage fuel and the second-stage oxidant. At least a portion of the second-stage plenum chamber is disposed within the first-stage plenum chamber and extends through the end wall, thereby providing fluid communication between the second-stage outlet and the combustion chamber.
In the method for combustion of a fossil fuel in accordance with one embodiment of this invention, a first-stage fuel and a first-stage oxidant are introduced into a combustion chamber and ignited, forming a primary combustion zone comprising first-stage combustion products. At least about 5% of the total heat output produced by combustion of the first-stage fuel and first-stage oxidant is removed from the primary combustion zone, forming cooled first-stage combustion products. A portion of the cooled first-stage combustion products is recirculated from a downstream region of the primary combustion zone to an upstream region of the primary combustion zone. A second-stage fuel is introduced into the combustion chamber downstream of the primary combustion zone, forming a secondary combustion zone and igniting the second-stage fuel, forming second-stage combustion products. At least about 5% of the heat from the secondary combustion zone is also removed. In accordance with one preferred embodiment of this invention, the first stage of oxidant comprises more than a stoichiometric requirement for complete combustion of the first-stage fuel.